White polyester films are widely used for applications such as a reflector and a reflection sheet of a surface illuminant apparatus in a flat-panel image display system used for liquid crystal display or the like, a rear-reflection sheet for an illuminated signboard and a back-reflection sheet for a solar cell because of characteristics that these film have uniform and high brightness and dimensional stability, and are low priced. As a method of exhibiting high brightness, there are widely employed methods of utilizing a difference in refractive indexes between inorganic particles contained in a polyester film and a polyester resin, or a difference in refractive indexes between minute voids and a polyester resin, such as a method in which a polyester film contains a great number of inorganic particles such as barium sulfate and light reflection at an interfacial surface between a polyester resin and a particle and a void's interfacial surface of the minute void produced with a core of particles is utilized (Japanese Unexamined Patent Publication No. 2004-330727), a method in which light reflection at a void's interfacial surface of the minute void produced with a core of a resin which is not soluble in polyester by fixing the resin which is, not soluble in polyester is utilized (Japanese Unexamined Patent Publication No. 4-239540), and a method in which light reflection at an interfacial surface of the void internally produced by including inert gas in a polyester film in a pressure vessel is utilized (International Publication WO 97/01117 pamphlet).
In recent years, particularly, applications in which liquid crystal display is used are remarkably expanded and the liquid crystal display is widely adopted for LCD televisions in addition to conventional laptop computers, monitors, and mobile devices, and in accordance with this, higher brightness and higher definition of a screen are required. There are requirements for high brightness and a high hiding property in reflecting sheets in response to the higher brightness of the screen. In accordance with these requirements, actions of increasing number of interfacial surfaces to reflect light in the polyester film, such as increasing an amount of inorganic particles in the polyester film and increasing an amount of a resin which is not soluble in polyester, are required, however there arises a problem that by increasing the amounts of inorganic particles and a resin which is not soluble in polyester, a film break often occurs during biaxial stretching and productivity is deteriorated, and it was difficult to achieve high brightness/high hiding property and the productivity of a film simultaneously. Further, the reflection sheet requires that its deterioration is suppressed and it can be used stably even if it is used for a long time. Hence, for example, in Japanese Unexamined Patent Publication No. 2002-098808, a void-containing polyester film containing a light stabilizer to secure the stability is disclosed.
Further, generally, a biaxially stretched polyester film easily produces static electricity and has a problem that dust adheres to the film during film forming and processing steps and when used. In recent years, since the above-mentioned dust has a large effect on production loss as a result of upsizing of a screen, a request for protecting each member from fouling during a production step of a liquid crystal monitor for which the polyester film is used be is increasing. For this request, an effect of preventing the fouling is achieved by providing an antistatic property for the film in Japanese Unexamined Patent Publication No. 10-278204. However, the film presented in Japanese Unexamined Patent Publication No. 10-278204 cannot be applied to a liquid crystal display device requiring light stability, and therefore an optical film which achieves light stability and an antifouling property simultaneously is required.
Further, on the other hand, species of resin which is not soluble in polyester are also studied (Japanese Unexamined Patent Publication Nos. 5-9319, 8-302048 and 2001-288284). However, it becomes difficult to respond to the high brightness and the high hiding property in recent years by technologies described in these Patent Documents, and further in Japanese Unexamined Patent Publication No. 2001-288284, the film is aimed to achieve cost and performance simultaneously by use of a cycloolefin copolymer and an ultraviolet stabilizer, but this film does not adequately achieve the suppression of the deterioration of reflectivity due to ultraviolet ray and screen brightness simultaneously.
It could therefore be helpful to provide a white polyester film achieving productivity and performance simultaneously, which achieves high brightness and a high hiding property simultaneously and hardly causes a film break and uneven luminance in a width direction, as well as achieving light stability and an antifouling property simultaneously.